Continuous casting is a technique for producing long cross-sectionally constant strands, or castings, such as slabs, billets, blooms, etc. These strands are produced by continuously pouring molten casting material, such as steel, through a mold in which it is allowed to harden partially before being drawn out.
The particular crystalline structure of the finished strand is affected by the freezing process in which a solidification front, or liquid metal/solid metal interface, traverses through the cooling casting material from the lower temperature regions to the higher temperature regions. Both the progression and velocity of the solidification front and its shape determine or affect the crystalline structure of the casting material and thus the mechanical characteristics of the final strand. Information regarding the exact profile of the interface is also important for optimizing magnetic stirring and soft reduction techniques employed to minimize phase segregation in high-alloy steel casting. Moveover, casting speed is limited largely by the location of the tip of the interface.
The solidification front, or the liquid metal/solid metal interface, progresses through the inside of the casting, since usually heat passes out of the object at its boundary with the environment. Consequently, it is very difficult to observe or even monitor.
A common method for predicting the profile and progression of the front is by computer estimation of heat flow throughout the strand. Unfortunately, the heat transfer phenomena associated with the process are very complex, and thus the position of the interface cannot be established at any one time with great accuracy. Moreover, since the calculations are complicated, automatic control of the variables in a continuous casting process can not be accomplished in real-time. Furthermore, rapid fluctuations in the interface and the instabilities associated with the freezing process can not be detected. Consequently, the conventional casting process is run on a semi-empirical basis such as by adjusting the rate of withdrawal of the strand, controlling the flow rate of coolant, and adjusting the temperature of the hot molten casting material. These procedures are run on a trial-and-error basis. As a result, it is not uncommon that many strands of sub-optimal microstructure and properties are produced and must be discarded or remelted.
Methods for detecting solidification fronts exist. These methods, however, are generally limited to non-metallic low melting temperature materials and cannot be used in the casting of high temperature metals and alloys on an industrial scale. The metal casting temperatures are very high and the processing speeds too fast.